Evaluation of the Impact of Out-Of-Phase Faults on Generators and Generator Circuit-Breakers implementing Vacuum Interruption Technology

Generator Circuit-Breakers (GCB), which are installed between the synchronous generator and the Generator Step-Up Transformer (GSUT), are not only used to interrupt the system source and generator source short-circuit currents but also to synchronize the generator with grids. During the process of the synchronization an Out-of-phase (Oop) fault can occur due to the wiring errors either from commissioning or from maintenance activities on the plant equipment. The angle of Oop decides the stresses on the equipment where a higher Oop angle leads to higher transient recovery voltages (TRV) and fault currents. At lower Oop angles, the DC time constant of the fault current will be longer that could lead to delayed current zeros. Thus, the stresses on the GCB while clearing the fault currents will be higher. This requires a GCB that is able to withstand longer arcing times and be interrupt these currents reliably.The vacuum interruption technology is well established in the medium voltage range up to 52 kV especially in the distribution circuits. The new dual logo standard IEC/IEEE 62271-37-013 (2015) describes the test procedure for testing generator circuit breaker with Out-of-phase faults and defines a maximum Out-Of-Phase angle of 90 °. The aim of this paper is to show the impact of Out-Of-Phase faults on generators and GCBs implementing vacuum interruption technology. Based on multiple simulations with different generator types using the commercial software PSS Netomac the impact of generator’s electrical & mechanical parameters on the Out-Of-Phase fault current behavior is studied. VGCBs have a special advantage in handling such Oop faults due to their capacity to withstand longer arcing times when compared to SF6. This makes VGCBs as a reliable technology in generator switching applications than other existing technologies today.